1. Field of the Invention
The present invention relates to a beam homogenizer for homogenizing intensity distribution of a laser beam in a particular region of an irradiated surface. In addition, the present invention relates to a laser irradiation apparatus for irradiating the laser beam to the irradiated surface. Furthermore, the present invention relates to a method for manufacturing a semiconductor device having a crystalline semiconductor film formed using the laser irradiation apparatus.
2. Related Art
In recent years, a method for manufacturing a crystalline semiconductor film by irradiating a laser beam to a non-single crystal semiconductor film formed over a glass substrate has been extensively employed (this method is hereinafter referred to as laser annealing). It is noted that the term “crystalline semiconductor film” herein used means a semiconductor film having a crystallized region, and includes a semiconductor film having the crystallized region all over the surface thereof.
The glass substrate is less expensive than a synthetic quartz substrate, thereby having an advantage that a large substrate can be manufactured easily. On the other hand, the glass has a disadvantage that it has a lower melting point than the synthetic quartz. However, when the laser annealing is performed to the semiconductor film formed over the glass substrate, it is possible to give high temperature only to the semiconductor film without giving any thermal damages to the glass substrate. In addition, the laser annealing provides much higher throughput than heating means using an electrically heated oven.
Since the crystalline semiconductor film manufactured by the laser annealing has high mobility, the crystalline semiconductor film is extensively used as an active layer of TFT for a driver circuit constituting an active matrix liquid crystal display device, for example.
A laser beam emitted from an excimer laser is often employed as the laser beam. The excimer laser has advantages that it has high output, that it can oscillate the laser beam at high repetition rate, and that the absorption coefficient of the laser beam emitted from the excimer laser is high to a silicon film, which is often employed as the semiconductor film. And, the laser irradiation is performed in such a way that the laser beam is shaped through an optical system so as to have a rectangular shape on the irradiated surface, and that the irradiation position of the rectangular laser beam is moved relative to the irradiated surface. Since such a method provides a high productivity, it is superior industrially. It is noted that the laser beam having a rectangular shape on the irradiated surface is referred to as a rectangular beam in this specification.
Since the laser beam emitted from the laser oscillator has Gaussian intensity distribution in general, it is necessary to homogenize the intensity distribution of the laser beam on the irradiated surface in order to perform the uniform laser annealing. In recent years, in order to homogenize the intensity distribution, a method is often employed in which a cylindrical lens array is used to divide the laser beam in a predetermined direction and then the divided laser beams are superposed in the same surface. According to this method, a rectangular beam can be formed to have a length of 300 mm or more in the major axis and a length of 1 mm or less in the minor axis, and therefore it has become possible to perform the laser annealing efficiently to the semiconductor film formed over a large substrate.
When the cylindrical lens array is used, however, a processing accuracy of each cylindrical lens is a problem. The cylindrical lens array consists of a plurality of cylindrical lenses arranged and it is impossible to manufacture the respective cylindrical lenses so as to have the same radius of curvature and the same accuracy in their surfaces. As a result, since the beam spots divided by the cylindrical lens array cannot be superposed in the same irradiated surface completely, there is a region in the rectangular beam to be formed where the intensity distribution is attenuated. This can be a problem when performing the laser annealing to the semiconductor film. When the semiconductor film formed by the laser annealing using the rectangular beam having such inhomogeneous intensity distribution is used to manufacture a TFT, and moreover when this TFT is used to manufacture a liquid crystal or organic EL display, a stripe or a color variation may appear on the display.